The invention relates to a practical method for commercially producing radiopharmaceutical activities and, more particularly, relates to a method for the preparation of about equal amount of Radon-211 (.sup.211 Rn) and Xenon-125 (.sup.125 Xe) including a one-step chemical procedure following an irradiation procedure in which a selected target of Thorium (.sup.232 Th) or Uranium (.sup.238 U) is irradiated. The disclosed method is also effective for the preparation in a one-step chemical procedure of substantially equal amounts of high purity .sup.123 I and .sup.211 At.
In many research applications it is desirable to have available the relatively long-lived radio-iodine isotope labels that have been found to be very useful in studying disease processes. In other applications, such as for therapeutic radiation dose treatment of certain human diseases, it has been found that the radionuclide .sup.211 At is very useful. It is also known that .sup.123 I is ideal for imaging in nuclear medicine, while .sup.211 At has desirable properties as a label of therapeutic radiopharmaceuticals that are used in the treatment of human diseases such as cancer and rheumatoid arthritis. Accordingly, it is recognized that a method for affording simultaneous production of about equal amounts of .sup.125 Xe and .sup.211 Rn would be of considerable value in making systematic investigations of the energetic and ionic reactions of .sup.125 I and .sup.211 At (the daughters of .sup.125 Xe and .sup.211 Rn, respectively) during excitation labeling of organic compounds. Moreover, conclusions concerning the chemistry of astatine are often drawn by extrapolation from iodine chemistry. Moreover, it is desirable to label organic compounds intended for biomedical studies with both iodine and astatine isotopes in order to ascertain biochemical behavior and in vivo stability. Thus, a method of preparing both the radionuclides (.sup.211 Rn and .sup.125 Xe) in a relatively carrier-free state is of value, because such extrapolations will thus be made more economically practical in view of the fact that with such a method the radiochemical yields can be optimized.
Before the development of the invention disclosed herein, it is not believed that any other processes or methods existed for the commercially practical, simultaneous preparation of substantially equal amounts of .sup.211 Rn and .sup.125 Xe. By practicing the method of the invention, such useful quantities of high radionuclidic purity, carrier-free .sup.211 At and .sup.125 I can be readily prepared. Accordingly, by the method of the invention, a single source containing both of those parent radionuclides is made available for dual-tracer preparation of radiopharmaceuticals, such as monoclonal antibodies.
Also, the method of the invention enables the preparation of high purity .sup.123 I and .sup.211 At, in the same chemical form and media, so that truly double-labeled compounds, which must be obtained in high specific activity for diagnostic and therapeutic applications, can be achieved. The chemistry of .sup.211 At is particularly difficult, because there are no stable isotopes of that element, so chemistry with .sup.211 At is generally based upon extrapolation from iodine chemistry. Thus, it is believed that the types of double-labeled radiopharmaceuticals, that can be economically prepared by practicing the method of the invention, will have future applications where the labeled compound can be administered to a patient, with the .sup.123 I label being used to locate a given desired site, such as the site of a tumor, for example, while the .sup.211 At is used for therapeutic treatment of the site. As indicated above, .sup.211 At does not have nuclear decay properties that would permit its use for imaging and, on the other hand, there are no alpha-emitting radionuclides of iodine, which would permit their therapeutic use.
Prior to the present invention, it was known that high purity .sup.211 Rn could be prepared by bombarding .sup.209 Bi with .sup.7 Li particles, for example, in a type of method such as that described in U.S. Pat. No. 4,364,898 which issued Dec. 29, 1982. However, that patent and related prior art methods do not disclose or suggest a method for simultaneously producing substantially equal amounts of radionuclides that are suitable for double-labeling compounds in the manner explained above.
In the applicants' co-pending U.S. patent application, Ser. No. 598,624, which was filed Apr. 10, 1984, there is disclosed a process for reliably producing a .sup.211 At radiopharmaceutical by a process that includes forming a suitable bismuth target and then irradiating it with alpha particles, preparatory to chemically treating the target to elute .sup.211 At, which is then collected in a controlled volume of eluent for use in selected radiopharmaceutical procedures. The disclosure of that co-pending U.S. patent application is referred to and incorporated herein by reference for its teaching of suitable techniques for forming radiation target bodies and target backing materials, as well as for the techniques described therein for irradiating such target materials.